A common practice in forming fibers of mineral material, such as glass, is to discharge molten glass from a forehearth into a rotating centrifuge or spinner. The molten glass flows across the spinner bottom wall to the spinner peripheral wall and passes in a molten state through the orifices of the spinner peripheral wall to create glass fibers. In some of the fiber producing apparatus, burners are positioned to direct flow of hot gasses or products of combustion into contact with the molten glass within the spinner and with the spinner bottom and peripheral walls of the spinner to maintain the glass at a sufficiently elevated temperature for proper fiberizing. These hot gases are then directed from inside the spinner to the exterior peripheral wall of the spinner to maintain the outside of the spinner at an elevated temperature. An additional high velocity blower is mounted about the periphery of the spinner directing high pressure air downward against the fibers to assist in attenuation. A gap or opening exists between the inside circumference of the high velocity blower and the outside circumference of the spinner. Ambient air is drawn downward through this gap by the action of the high pressure air from the high velocity blower and the hot combustion gasses exiting from the top of the spinner. The ambient air flow, however, tends to be turbulent causing the fibers to undulate, hit into one another, deform the diameter and stick together if they have not passed into the solid state. The ambient air also causes instability of the veil of fibers falling on the collection devices. It has been found that the addition of a flow controller over a portion of the air gap stabilizes the ambient air flow allowing uniform fiber production and a stable fiber veil shape.